An “inceptor” is a device that allows a human operator to control a machine. Examples of inceptors include the stick in an aircraft, a steering wheel in a car, the gloves sometimes used for robotic arms, or a joystick in a crane or other piece of construction equipment. An “active inceptor” means that a motor can move the stick/wheel/glove/joystick to provide feedback to the human operator.
The inceptor works by sensing or receiving the force applied by the human operator, then manipulating the machine accordingly. The displacement on the inceptor controls the machine. Whereas a mechanical system's force-versus-displacement and force-versus-velocity characteristics (i.e., passive inceptors) cannot be varied easily, using an active inceptor allows essentially instant reconfiguration of these characteristics. This allows the computer to add tactile cueing features like “soft stops” that indicate precisely and intuitively the position beyond which the inceptor should not be moved. A soft-stop introduces a relatively large incremental change in inceptor force beyond a specific inceptor position that a human operator perceives as a solid feeling stop. Although the incremental force change with position is relatively high at the soft stop, perhaps 5 pounds or so of incremental force change over a fraction of an inch of inceptor motion, the absolute value of inceptor force required to overcome the soft stop is still relatively low, perhaps 5 pounds or so of absolute force. Although a human operator can easily push past a soft stop when necessary, a properly designed soft stop eliminates the possibility of inadvertent movement of the inceptor beyond an operational limit. When a human operator intentionally pushes the inceptor through a soft stop, the relatively low level of soft stop force continues to cue the operator that a limit is being exceeded, but the level of soft stop force is low enough that the human operator can position the inceptor precisely with relatively low levels of muscular fatigue. Hard stops use the full force-generating capability of the motor, perhaps as much as 50 to 200 pounds of force, in an attempt to stop the operator from moving the inceptor any further when the computer identifies that a catastrophic failure may result.
As an example, motor vehicle steering wheels have traditionally been passive inceptors. The steering wheel may be turned until the mechanical limit for turning the wheels has been reached. The steering wheel may also kick when a vehicle drives over a pothole. The feedback is limited to the activity of the mechanical system. If the steering wheel is an active inceptor, it could be programmed to further limit turning of the steering wheel dependent upon the velocity of the motor vehicle to prevent rollovers. When the human-operator is not applying any force, the computer-calculated forces may provide feedback to the inceptor and cause the inceptor to move. Allowing the computer the ability to move the inceptor introduces a safety risk.
Active inceptors for aircraft including helicopters have been in development for years, but are just beginning to be introduced. For example, as applied to helicopters, active inceptors (or “active sticks”) replace the helicopter's cockpit control springs and dampers, which otherwise provide “force-feel” on the collective and cyclic sticks, with electric motors and a computer to allow varying force on the stick. The active inceptor system allows the computers to communicate with the pilot through “tactile cues” in addition to the existing methods of cockpit displays and aural warnings. By moving communications from the cockpit displays to the pilot's hands, the pilot may be able to keep focus on activity outside the cockpit for safety and mission effectiveness.
However, these active inceptors must be able to move very fast and through the full range of travel. When the operator has his hands firmly on the controls, failure modes of active inceptors are fairly benign because the operator instantly senses the change in inceptor force caused by the failure and acts instinctively and biomechanically to inhibit undesirable movement of the inceptor. For typical aircraft applications, computer-controlled fast-moving actuators act in series with pilot inputs and have a limited range of travel to ensure failure robustness. Series actuators add or subtract control surface actuator motion “in series” with the control surface motion commanded by cockpit control inceptor inputs. Thus series actuator inputs do not result in motion of or forces exerted on the cockpit control inceptors. Series actuators behave conceptually like an “extensible link” in the path from cockpit controls to control surfaces. The series actuator or “extensible link” can extend or retract independently to move the control surfaces while the cockpit controls remain stationary. In contrast, parallel actuators exert force on the cockpit control inceptors manipulated by the pilot; hence they are referred to as “parallel” actuators because they exert forces on the control inceptors in parallel to the forces exerted by the pilot. Actuators with large travel typically act in parallel with pilot inputs and have limited rate capability to ensure failure robustness. This relationship between actuator speed and travel is a safety feature intentionally designed into aircraft such that no single computer-controlled actuator can cause excessive and unrecoverable vehicle motion before a human operator intervenes. In the case of Fly-By-Wire systems, high-bandwidth and full-authority swashplate actuators are limited in the control software to reproduce the limited-travel plus limited-speed safety feature. No such limiting has been applied to active-inceptor technology to address sensitivity to failures in hands-off operating conditions. Current active-stick technology development focuses on the advantages of the technology, not the safety features necessary for a production system.